The invention relates to audio recorders which are utilised in parallel with video recorders for the purpose of recording an audio signal on the one hand and a video signal on the other hand, on two separate magnetic tapes. Recording the audio signal on another magnetic tape than that of the video recorder permits securing an improved sound quality. For editing the sequences, the audio and video recordings have to be located and synchronised. The SMPTE and EBU organisations have established a standard specifying a digital code and a modulation method permitting reference and synchronising signals to be recorded on the video and audio tapes.
According to this standard, each video image (two frames) is associated with a code word comprising 80 bits, formed by temporal data bits and by control bits. The temporal data bits code the tens of hours, hours, tens of minutes, minutes, tens of seconds, seconds, tens of images and images. The last 16 bits of the code word form a synchronising word which permits the referencing of the code word and detecting the direction of travel of the tape.
The standardised modulation method is such that a first logic level transition appears at the start of each period of bits. In the case of an O, there is no second transition during the period of the bit. In the case of an I, a second transition appears half a period after the start of the bit.
On a video tape, the code word may be recorded on a track separate from the video track and referred to as the audio or "cue" track 3. Another method consists in recording it on the video recording track, during the frame suppression interval separating each image from the following image. On a magnetic audio tape, the code word is recorded on a track separate from the audio tracks. During playback, the decoding of the code word is performed by means of known devices permitting reading and decoding irrespective of the running speed of the magnetic tape between one fifth and a hundred times the nominal playback speed, whether on the video tape or on the audio tape.
In the case of stopping on an image, the speed of the video tape is nil, the reading of the code word is impossible unless it is recorded within the frame suppression interval. In this case, the reading or playback is performed by the magnetic head performing the reading of the video signal; this head being installed on a rotary drum, the reading of the code word occurs whilst stopped on the image, in the same manner as the reading of the video signal. If the code word is recorded on the audio track 3, the reading of the code word is possible only whilst the tape is in translatory motion at a speed equal to at least one fith of the scheduled speed, and the same applies to reading the code word on the magnetic audio tape. In the case of a video recorder the problem is resolved by recording the code word within the frame suppression interval, whereas, on the contrary, there is no method of resolving this problem for a tape recorder.
The impossibility of reading the temporal code of the audio tape during a stop on the image has the result that it becomes difficult to synchronise the sound and upon image editing the sequences.
The audio recorder according to the invention resolves this problem by means of a moving magnetic head permitting reading of the code word irrespective of the speed of the magnetic tape, even if this speed is zero.